PRIOR ART FIG. 1 shows the pixel structure of a striped liquid crystal display (LCD 100), which is well known in the art. When a first and second display layers are stacked, moiré interference is produced. The interference is caused by interactions between the color filters within the layers when projected onto the viewer's retina. For example, when green color filters overlap, light is transmitted making for a comparative bright patch. When a green filter is over say a red filter, not as much light will be transmitted making for a dark region. Since the rear and front display layers have slightly different sizes when projected onto the retina, the pixels will slowly change from being in phase to out of phase. This has the effect of producing dark and bright bands otherwise known as moiré interference.
There are several approaches to removing moiré interference in an MLD system. Most approaches rely on removing unwanted frequency components by spatial filtering. This can be accomplished with either a diffuser type system whereby an element with a refractive index of ˜1.5 has random surface perturbations, or a diffraction type system. The performance of these systems in terms of visual aesthetics (e.g., how blurry the image looks; how much residual moiré is left; the effect on polarization; and cost, etc.) depend greatly on the system configuration.
Current multi-layered display (MLD) systems utilize diffusive optics to blur the rear most display layer. While commercially successful, this approach suffers from the following limitations: (a) the rear most image is inherently blurry—there is a trade-off between reducing moiré interference and the clarity of the rear most image display layer; (b) the diffusing element utilizes a specialized diffuser pattern, which is difficult to obtain; (c) the diffusing element sits between polarizers and both the film substrate and stiffener substrate must be free of any birefringence; and (d) the diffusing element requires a separate stiffener component (usually glass) which adds weight and expense to the final display system. As a result, diffusive type systems do not provide an ideal solution to reducing moiré interference in MLD systems, especially as those systems have reduced form factors.
In a diffraction type system of the prior art, to prevent interference from the color filters, several copies of an image are required, wherein the number of copies is defined as the rounded ratio of the width of the pixel to the width of the sub-pixel. However, while the diffraction grating is configured to generate copies of the image properly, moiré interference from the black matrix masking associated with electronic traces to each pixel is not alleviated.
Further, a disadvantage of the diffraction type system solution is that multiple orders are difficult to generate simultaneously, since this requires multiple periods PRIOR ART FIG. 2 shows the efficiencies of various blazed gratings for a diffraction type system implemented to reduce moiré interference. As can be seen from PRIOR ART FIG. 2, phase gratings with simple repeating structures are only efficient at producing zero and first order diffraction simultaneously. Higher orders, including second order and third order copies, are not shown to be generated simultaneously with the first order.
What is desired is an MLD system that addresses the moiré interference due to overlapping black matrix masking from multiple display layers.